1. Field of the Invention
The present invention relates to an Fe--Cr--Al alloy foil having high oxidation resistance at elevated temperatures, which is suitable for a substrate of a catalytic converter.
2. Description of the Related Art
Catalytic converters are used to remove harmful elements, such as NO.sub.x, CO or HC, generated by burning fossil fuel. Various types of catalytic converters are used in automobiles. Of such converters, converters composed of a honeycomb-structure made from an Fe--Cr--Al alloy foil have come to be widely used, because they have many advantages, such as reducing pressure loss.
A catalytic converter made from an Fe--Cr--Al alloy foil is disclosed in, for example, U.S. Pat. No. 4,318,828. This publication proposes that Cr is 15 to 25 wt. % Al 3 to 6 wt. % and Y 0.3 to 1.0 wt. %. However, since Y is a rare metal and expensive, alloy foils containing it become very expensive, so that it is difficult to use these foils in a common automobile from an economical point of view.
On the other hand, it has been proposed in U.S. Pat. No. 4,414,023 that Cr: 8 to 25 wt. % Al: 3 to 8 wt. %, and rare-earth elements: 0.002 to 0.06 wt. % be used. In this publication, rare earth elements (in particular, Ce and La), which are cheaper than Y, are used in place of Y. Rare-earth elements are added to improve the adhesion of surface oxide to metal. It is described that these rare-earth elements should not be added in an amount in excess of 0.06 wt. % because they degrade the hot workability of alloys. This alloy is less expensive than the alloy foil of the above-described U.S. Pat. No. 4,318,828. However, it encounters a problem in that it has inferior oxidation resistance. It is a matter of course that the oxidation resistance is a more important property for a foil used in a catalytic converter.
In connection with this, when the inventors of the present invention examined in detail the influence of rare-earth elements, in particular La, Nd and Ce on the oxidation resistance and the hot workability of the alloy, they found that when about 0.05 or more wt. % of La and Nd are added, the oxidation resistance is improved to such an extent that it is comparable to alloy foils containing Y, and found that Ce should be removed as much as possible because Ce is the main factor resulting in degradation of the hot workability of alloy.
Using these results, the inventors of the present invention proposed in U.S. Pat. No. 4,904,540 an alloy to which La is added, and in U.S. Pat. No. 5,228,932, an alloy to which Nd is added. Further, it is described in U.S. Pat. No. 5,228,932 that the addition of a mixture of La and Zr improves the oxidation resistance of the foil. These alloys containing La, Nd and Zr exhibit higher oxidation resistance that are better than those of an ordinary Fe--Cr--Al alloy foil. It has come to be widely used.
However, as exhaust gas regulations have been recently tightened, there has been a demand that the catalytic converter is installed near the engine. Thus the foil is subjected to a more severe high-temperature oxidation. A foil having even better oxidation resistance of alloys is required. To further increase the oxidation resistance in the components in the above-described U.S. Pat. No. 5,228,932, it is necessary to increase the content of Al.
However, when the inventors of the present invention conducted research using usual stainless steel manufacturing facilities, it became clear that cracks occur in the edge of a hot rolled coil if the content of Al exceeds about 6 wt. % in an alloy containing both La and Zr, and with such cracks as the origin, the coil tends to be broken frequently during cold rolling, and the yield becomes low.
Similar problems arise in an alloy disclosed in Japanese Patent Laid-Open No. 3-36241 in which La, Ce, Zr and Hf are added.
We have examined the influence of alloy elements upon hot workability in more detail, and we have discovered that an alloy in which Sm is added in place of La has good hot workability, so that cracks do not occur even in the edge of coil, and that the alloy can be manufactured without problems by using the usual stainless steel manufacturing facilities. Similarly, we have thoroughly examined the oxidation resistance of the foil and have found that an addition of Sm, Hf and Zr considerably improves the oxidation resistance of an alloy foil. Further, we also have found that annealing under appropriate conditions improves the oxidation resistance of the alloy foil.